1. Field of the Invention
The present invention relates to a technology capable of satisfactorily preventing stress corrosion cracking of a housing fastened, by welding, to an end bracket in the lower portion of a reactor pressure vessel.
2. Description of the Related Art
A variety of housings penetrate an end bracket in the lower portion of a reactor pressure vessel, the housing being secured by welding.
An example will now be described with reference to a case of an in-core monitor housing.
As shown in FIG. 16, an end bracket plate 1 in the lower portion of the reactor pressure vessel is composed of a body member 1b of the end bracket 1 and an inner clad portion 1a applied to the inner surface of the body member 1b, the inner clad portion la being made of a material exhibiting excellent corrosion resistance.
Hitherto, as shown in FIG. 16, the in-core monitor housing (hereinafter simply called a "housing") and the end bracket plate 1 of the reactor pressure vessel have been joined with each other in such a manner such that a cylindrical housing 3 made of SUS304 stainless steel is inserted into a through hole formed in the end bracket plate 1 and then the outer surface of an intermediate portion of the housing 3 is subjected to a circumferential weld with an inside clad 1a of the end bracket plate 1.
However, an undesirable sensitized zone 8a, the corrosion resistance of which is deteriorated due to an affection of weld heat, is continuously formed in the portion of the housing 3 adjacent to the circumferential weld 2 in the circumferential shape.
A tensile residual stress, the level of which reaches the yield point of the material, is sometimes present in the vicinity of the weld portion. Furthermore, when the nuclear reactor is operated, high temperature and high pressure primary water in the reactor pressure vessel also comes in contact with the outer and inner surfaces of the housing 3 higher than the circumferential weld 2. Therefore, triple factors, that is, a corrosive environment, reduction in the tensile stress and deterioration of the corrosion resistance are able to be simultaneously present in the circumferential weld 2 and its adjacent portions.
The above-described simultaneously-present factors will cause a stress corrosion cracking to be easily generated in the portion of the housing 3 in the vicinity of the circumferential weld. Furthermore, resistance against corrosive environment such as that against the corrosion fatigue will be deteriorated, causing the safety of the housing made of SUS304 to be critically deteriorated.
FIG. 17 illustrates an example of a housing the inner surface of which is applied with a surface reform treatment for the purpose of improving the portion in which the above-described three factors have been present.
That is, a surface reformed portion 9 is formed on the inner surface of the housing 3 so that the sensitized zone 8a shown in FIG. 16 cannot come in contact with primary water in the reactor pressure vessel in the housing. As the surface reform treatment, there has been available a method which is arranged in such a manner that the inner surface of the sensitized zone 8a, which has reached the inner surface of the housing, is simply remelted by using a nonconsumable electrode such as a TIG welding electrode without the filler metal. As a result, the sensitized zone 8a is given high density energy so that the sensitized structure is improved. Another method can be employed in which high corrosion resistant metal is clad-welded to the inner surface of the sensitized zone 8a. The above-described methods have been disclosed in U.S. patent application Ser. No. 07/445,535 and 07/561,684. However, even if the above-described remelting method or the clad-welding method are employed to reform the surface, a sensitized zone is formed in vertical end portions 10a and 10b of the reformed portion due to the heat affection at the time of the remelting operation or the clad welding operation although its degree is not considerably large. As a result, the tensile residual stress is undesirably generated.
Accordingly, there is a desire to improve the corrosive environment of the housing 3 while eliminating the remelting work or the welding work from the housing 3.
On the other hand, the inner surface of the housing 3 and the outer surface of the same which is higher than the circumferential weld 2 are positioned in contact with primary water in the reactor pressure vessel. Therefore, there is a desire to improve the corrosive environment of the above-described two surfaces.
Furthermore, the portion of the housing 3 of the reactor pressure vessel higher than the circumferential weld 2 is submerged in primary water in the reactor pressure vessel and primary water is contaminated and its quantity is great. Therefore, there is a desire to improve the corrosive environment of the two sides of the housing without removing primary water of the great quantity. Therefore, there is a desire to subject the housing 3 to a work for improving the corrosive environment performed from the inside portion of the housing 3.
As described above, since the housing of the reactor pressure vessel is used under special conditions, a multiplicity of requirements must be met when the work for improving the heat affected zone is performed.
Therefore, it might be considered feasible to employ technologies disclosed in Japanese Patent Application Laid-Open No. 60-131923 (1985) and Japanese Patent Application Laid-Open No. 60-135526 (1985).
According to Japanese Patent Application Laid-Open No. 60-131923 (1985), a technology has been disclosed about a duplex tube structure arranged in such a manner that an intermediate portion of the outer surface of an inner tube is welded to the end portion of the outer tube, the technology being characterized in that: the duplex tube is heated by a heating coil disposed in the inner tube, a cooling medium is allowed to pass through the inside portion of the inner tube and the cooling medium is sprayed to the outer surface of the outer tube so that the compressive residual stress is generated on the inner surface of the inner tube and the outer surface of the outer tube which are positioned in contact with the above-described cooling medium.
However, in a case where the housing is sealed so as to prevent the leakage of primary water in the reactor pressure vessel through the housing, a state in which the cooling medium passes through the inside portion of the housing cannot be realized. Therefore, the above-described technology cannot be preferably applied to the housing of the reactor pressure vessel. Furthermore, the technology of generating the compressive residual stress on each of the inner and the outer surfaces of the housing by the heat treatment performed from the inside portion of the housing of the reactor pressure vessel while remaining the primary water in the reactor pressure vessel has not been disclosed. In addition, the above-described technology includes no method and means for using primary water in the reactor pressure vessel as the cooling medium. According to the above-described technology, heating and cooling of the inner surface of the inner tube are simultaneously performed at the time of the heat treatment process. Therefore, it has been difficult to obtain the temperature difference which is sufficiently large to yield the housing in terms of the stress even if the above-described heat treatment technology is applied to the housing.
According to Japanese Patent Application Laid-Open No. 60-135526 (1985), a technology has been disclosed about a duplex tube structure arranged in such a manner that an intermediate portion of the outer surface of an inner tube is welded to the end portion of the outer tube, the technology being characterized in that: the duplex tube is heated by a heating coil disposed in the inner tube and the inner surface of the inner tube in a region which overlaps the outer tube and the outer surface of each of the inner tube and the outer tube are cooled by the cooling medium sprayed. As a result, the compressive residual stress is generated in the surfaces which are positioned in contact with the above-described cooling medium. According to this technology, the compressive residual stress is not generated in the inner surface of the inner tube in the region which does not overlap the outer tube with respect to the weld portion.
According to the above-described technology, the method and means of heat treatment in which primary water in the reactor pressure vessel is utilized as the cooling medium has not been disclosed. Furthermore, the technology has not been disclosed in which the compressive residual stress is generated in the overall region of the heat affected zone of the welded portion because the compressive residual stress remains in the inner surface of the inner tube of the region which does not overlap the outer tube with respect to the welded portion. In addition, according to the above-described technology, since heating and cooling of the inner surface of the inner tube are simultaneously performed, the temperature difference which is satisfactorily large to yield the housing in terms of stress cannot be easily obtained between the inner surface of the housing and the outer surface of the same even if the above-described heat treatment technology is applied to the housing of the reactor pressure vessel. Furthermore, the temperature difference which is sufficiently large to yield the housing in terms of stress cannot quickly and assuredly obtained.